Mixed-signal circuits, defined as circuits containing both analog and digital circuits, are plagued by injection of noise from the digital circuits into the sensitive analog circuits. Such noise, commonly known as cross talk, produces errors in the monitored analog information. One major source of digital noise is the embedded memory circuits commonly used in such circuits, as well as the data and control signals to and from the embedded memory circuits.
In addition, the analog circuits are typically temperature-sensitive and their operation is affected by heat generated during digital switching activity. The time constants of thermal events in semiconductor substrates are often on the same order as the time for the circuit to perform sensitive analog operations. Thus, analog performance degrades if chip temperatures vary with varying digital switching activity.
Various techniques for isolating digital noise from the analog circuits have been developed. These include separation of analog and digital power supply and ground lines and isolation of transistors in the digital portion of the circuit within electrically isolated diffusion wells. U.S. Pat. No. 5,793,318 to Jewett describes a system for preventing distortion in an analog-to-digital converter by XORing each bit of the generated digital data with a different pseudo-random bit to generate an encoded digital output signal. This removes all correlation between the analog input signal and the digital output signal.
Such measures for mitigating the effects of digital noise can limit the design and layout of components in integrated circuits as well as increasing integrated circuit complexity.
It would be desirable to have mixed-signal circuits that do not suffer from the above disadvantages.